A door-spring is often provided in a known manner with a blocking device, which is used to counter the force exerted by a door-spring and secure a door open. By applying a blocking device on a door provided with a door-spring, it is possible to hold a door in an opened position for a longer time without human intervention. A known blocking device is, for instance, constructed such that a door-spring blocked by this device can be released by human intervention. This human intervention includes, for instance, applying a small force to a blocked door in the direction in which the door is carried from an opened to a closed position.
A known release mechanism of the type referred to above can also be activated automatically, that is, without human intervention. Such a release mechanism is coupled, for instance, to a smoke detector and is applied, for example, in combination with the door-springs of doors which are closed automatically in the case of a smoke and/or fire alarm.
A release mechanism disclosed in U.S. Pat. No. 3,371,823 includes, for instance, the combination of a blocking device and an electromagnet. In the situation where the blocking device blocks a door-spring, the electromagnet is energized and actuates a biased control member of this blocking device. When the current which energizes the electromagnet falls away, the control member can be displaced by the exerted bias.
Such a release mechanism has a number of drawbacks. The release mechanism cannot be used without a control circuit for supplying the current for the electromagnet. The power consumption of such a combination is comparatively high which is undesirable, particularly in large buildings with a large number of doors provided with such a release mechanism.